The present invention relates generally to methods of switching electrical current, and to switching methods that are particularly suitable for changing the direction of current flow in a large electrolytic cell, though the invention is not limited thereto.
In the operation of electrolytic cells employed to produce aluminum by electrolysis of alumina disposed in a fused salt bath, before the alumina and bath are placed in the cell, the cell is usually preheated to a predetermined, operating temperature. Heretofore, this has been accomplished by a variety of ways, one of which has been the use of resistant heaters located in the cell or in the walls of the cell, such as shown in U.S. DeVarda Pat. No. 2,959,528. In addition, the DeVarda patent discloses the use of a certain level of preheated molten metal located between electrodes located vertically in the cell and conducting current through the electrodes and molten metal as a means to preheat the cell, the interface of the molten metal and the electrodes being somewhat resistant to the flow of current.
In U.S. Tilson Pat. No. 1,572,253, a Hall cell is preheated by lowering the anodes of the cell into physical contact with the cathode liner thereof, and applying direct current across the anodes and liner, the interface of the anodes and liner being resistant to the flow of current such that the liner and cell are heated.
In U.S. DeVarda Pat. No. 3,382,166 there is shown a two-stage heating process for a multicell furnace having a large plurality of electrodes suspended vertically in the furnace and a corresponding plurality of vertically extending bars. The second stage of the two-stage process involves connecting each two adjacent bars together, with a conductive latch or bar, in a series or parallel arrangement. Current is then directed through the electrodes to preheat the cell. After an appropriate temperature is reached, each connecting bar or latch is removed to return the cell to an operating, metal-producing condition.